1. Field of the Invention
The present invention generally relates to imaging apparatuses and methods, and more particularly to an imaging apparatus and method that are suitable as and for a laser printer, a digital copier, or a facsimile apparatus having a writing unit that performs image exposure on a photosensitive body by deflecting a light beam by using a deflector.
2. Description of the Related Art
Recently, imaging apparatuses such as printers have been designed to have a higher image density so that printers that can perform writing at 1200 dpi have been put into practical use. On the other hand, most of the commercially available conventional digital copiers having a printing function have a resolution of 600 dpi as a copying function. It is considered that a complex machine of a 1200-dpi printer and a 600-dpi digital copier can be realized without changing the number of rotations of a polygon mirror and the frequency of a print pixel clock signal by printing a copied image of 600 dpi by outputting the same data for each of 2×2 dots in a primary or main scanning direction and a secondary or sub scanning direction at 1200 dpi. This method is employed, for instance, in printing an image of 300 dpi by the conventional 600-dpi printer. A specific method of realizing this method is disclosed, for instance, in Japanese Laid-Open Patent Application No. 8-9119.
Further, Japanese Laid-Open Patent Application No. 9-275494 discloses a method of increasing the number of gradations by doubling image density in the primary scanning direction and assigning different data to each pixel having the doubled image density.
Generally, the frequency of a print pixel clock signal is proportional to the product of writing densities in the primary and secondary scanning directions. Therefore, given the same linear velocity, the frequency of the print pixel clock signal is four times as high in a printer engine of 1200×1200 dpi as in a printer engine of 600×600 dpi. For instance, if a printer of 600 dpi of approximately 20 ppm has a print pixel clock signal frequency of 25 MHz, the printer is required to have a print pixel clock signal frequency as high as 100 MHz to serve as a printer of 1200 dpi.
Laser diode (LD) multi-level modulation is realized by the above-described various methods. However, as the frequency of the print pixel clock signal becomes higher, it becomes more difficult to increase the number of modulation values. A method of performing pulse-width modulation (PWM) by using a high-speed clock signal using phase-locked loop (PLL) is well-known, for instance. It is known that this method may be realized by generating, inside an IC, a clock signal having a frequency of 400 MHz by using PLL and outputting from the clock signal a pixel clock pulse signal of 100 MHz that is pulse-width-modulated to have a one-fourth resolution. In this case, the multi-level or grayscale resolution of one dot in the case of writing at 1200 dpi can be selected from five ¼-apart pulse widths of 0, 1/4, 1/2, 3/4, and 1. Therefore, the method is a PWM method with five values.